The present invention relates to anion adsorbents capable of adsorbing various anions that are harmful to human and environment (negatively charged ions, examples of which include inorganic or organic anions, such as phosphate, arsenate or fluorine ions) with high efficiency. More specifically, the present invention relates to anion adsorbents useful as water or soil purifiers for removing such harmful anions from water or soil, for example.
In recent years, a phenomenon, so-called eutrophication, which is attributable to a rapid increase in burden of nitrogen and/or phosphorus, has been a concern in view of environmental conservation measures. As countermeasures to phosphoric acid as a factor of eutrophication, various phosphorus adsorbents such as those composed of granulated coal ash and those produced by mixing and firing volcanic ash and ferrous sulfate have been proposed, as described in Patent References 1 and 2.
Further, with respect to arsenate ion (ASO43−) and arsenite ion (AsO2−) that are toxic anions present in water, the water quality standard was revised in 1993 to a more stringent value of no more than 10 μg As/1. As such, techniques for enabling highly efficient removal of arsenic in water are needed to be established. As a process for removing arsenate and arsenite ions in the prior art, such a procedure is known that metal salts, such as calcium, iron and aluminum salts are added to arsenic-containing water to produce hydroxides of such metals and then the arsenic oxide is coprecipitated with the metal hydroxides for removal (see the section “Prior Art” of Patent Reference 3).
Patent Reference 1: Japanese Unexamined Patent Publication No. 2004-113885
Patent Reference 2: Japanese Unexamined Patent Publication No. 2004-298668
Patent Reference 3: Japanese Unexamined Patent Publication No. 1997-327694